1. Field of the Invention
This invention relates to centrifugal couplers and particularly to centrifugal clutches that use magnets to transfer the torque between an input shaft and an output shaft.
2. Description of Background
Centrifugal frictional clutches have been used for many years. In general torque of an input shaft is transmitted to an output shaft through the centrifugal clutch. The torque transmitted from the input to the output shaft is a function of the rotational speed of the input shaft, such that the torque transmitted increases as the speed increases. Centrifugal clutches, therefore, by design have slippage of frictionally engaged members as the speed of the output shaft is accelerated to the speed of the input shaft. This slippage of the frictional members creates wear that limits the operational life of the clutch. Additionally, the wear caused debris that can contaminate bearings and effect the operational life of the bearings as well.
Magnetic couplers are also used in the art for torque transmission between an input shaft and an output shaft. Magnetic couplers have an advantage of near infinite life since no parts are frictionally engaged during the torque transmission. Additionally, magnetic couplers allow for physical isolation between rotating parts, and are therefore commonly used in pumping applications of liquids. One example is in the pumping of cooling liquids in computers where the immersion of bearings can result in the need for frequent maintenance. Typically magnetic couplers are in full torque engagement at all times and are therefore transmitting their maximum torque at all times regardless of the speeds of the input shaft and the output shaft.
A combination of the two aforementioned concepts are combined into a magnetic centrifugal clutch, that is also known in the art and is described in detail in U.S. Pat. No. 5,691,587 (hereafter '587), included herein in its entirety by reference. By mounting the magnets to the input shaft in a radially slidable manner and biasing the magnets radially inwardly, '587 allows the magnetic centrifugal clutch to transmit substantially no torque from the input to the output shaft when the rotational velocity of the input shaft is low. A low torque transmission during start up of the input shaft is desirable when a sensorless brushless direct current (SBLDC) motor drives the input shaft. This is due to the fact that SBLDC motors posses very little starting torque. However, the magnetic centrifugal clutch of '587 prohibits the fluidic isolation of the input shaft from the output shaft. For pump applications this is problematic since bearings and seals are common sources of leaks.
Accordingly, there is a need in the art for a clutch that transmits a variable torque from an input shaft to an output shaft and simultaneously permits the input shaft to be fluidically sealed relative to the output shaft.